1. Field of the Invention
This invention relates to fluidized catalytic cracking process and apparatus for resid in general and heat integration of reaction-regeneration sections of the resid FCC unit in particular.
2. Description of the Related Art
Prior Art
Fluidized catalytic cracking (FCC) is one of the most important conversion processes in the refining industry. FCC was initially designed for a silica-alumina matrix type catalyst with a dense bed reactor-regenerator system. However, since the introduction of zeolite type catalyst, the FCC reactor has been converted to all riser cracking with significant reduction in riser residence time and catalyst inventory.
With further improvement in the catalyst composition, FCC could be run at higher metal level (5600-7000 ppm Nickel and Vanadium) on an equilibrium catalyst. Simultaneously, for reduction of bottom of the barrel, conventional FCC units were modified for handling heavy residue, e.g., atmospheric and vacuum resid, etc. The modification involved the improvement in feed atomization, quick riser termination and quench, better catalyst stripping, two stage catalyst regeneration, external catalyst cooler, catalyst and air distributors etc.
In a resid FCC unit, the feed is preheated to 150-250.degree. C. and injected radially at the bottom of the riser with steam as a dispersing medium. The contact time of the riser is kept in the range of 2-6 secs and the temperature in the riser bottom and top normally remains around 540-580.degree. C. and 500-540.degree. C. respectively. Suitable riser terminator devices are attached at the end of the riser to quickly disengage the catalyst from the product vapor. The catalyst is guided to a bubbling bed stripper where steam at the rate of 2-5 kg/1000 kg of catalyst is injected at the bottom of the stripper to remove the entrapped hydrocarbon vapor from the catalyst. The product vapour after the riser terminator is quenched or guided to the second stage cyclone and finally to the main column fractionator. The stripper catalyst is fed to the 1st stage of regenerator which works in the temperature range of 650-690.degree. C. The carbon on catalyst is significantly reduced (70-80%) in this stage which then is pneumatically conveyed to the second stage regenerator where the temperature is kept much higher (710-740.degree. C.) with sufficiently excess oxygen for near complete removal of carbon (&lt;0.05%) on catalyst. The regenerator catalyst from the second stage of the regenerator is fed to the riser bottom through a regenerated catalyst slide valve where the catalyst circulation rate is controlled to maintain the riser top temperature. Typically the resid FCC unit operates at a 5-8 cat/oil ratio. In some resid FCC units where the quality of resid (Conradson cokes 3-4%), the catalyst in the regenerator is cooled in an external catalyst cooler to maintain over all heat balance of the unit.
Although many modifications in the original FCC unit have been made earlier to process residues, such resid FCC units can not handle very heavy residues where the Conradson carbon is more than 6-8% and metal level (Ni+V) on feed is higher than 30-50 ppm. Several problems are associated in the known resid FCC units to economically process resid. These problems are as follows:
i) Excessive coke with the resid produces a large amount of excess heat and therefore the heat balance of the reactor regenerator is disturbed. PA1 ii) Higher metal level on the resid leads to significant deactivation of the catalyst and requires a very large catalyst addition rate to keep the metal level on equilibrium catalyst in an acceptable range. PA1 iii) Crackability of some of the residue, in particular aromatic residues, are not quite good. Sufficient residence time for such residues are required in the riser and the extra coke generated from such aromatic residue cracking is required to be handled. PA1 iv) Poor strippability of the catalyst: Strippability of the heavier unconverted residue inside the catalyst pores is not at all efficient. PA1 v) SO.sub.2 emission from present resid FCC units are very high and present resid FCC conditions are not very conducive for efficient functioning of SO.sub.x removal additives. PA1 vi) NO.sub.x generation in present resid FCC unit is quite high due to high temperature regeneration. PA1 i) Higher regenerator temperature reduces the catalyst circulation rate for a given riser top temperature to maintain in the reactor heat balance. Thus, the effective cat/oil ratio drops significantly resulting in reduced conversion. PA1 ii) Higher regenerator temperature significantly increases catalyst deactivation both due to the metal, as well as hydrothermal factors. In fact, a regenerator temperature beyond 700.degree. C. exponentially increases the zeolite crystallinity loss which is further aggravated in the presence of vanadium impurities on catalyst. The maximum vanadium level which can be tolerated in the FCC depends on the regenerator temperature. The tolerable vanadium level can be significantly improved by 4-5 times if regenerator temperature is reduced from say 730.degree. C. to 680.degree. C. Similarly, hydrothermal deactivation of catalyst also drops significantly with regenerator temperature reduction. PA1 iii) Higher regenerator temperature is not conducive for SO.sub.x additive which works better at moderate regenerator temperatures (680-700.degree. C.). Similarly, NO.sub.x emission is significantly increased beyond regenerator temperature of 720.degree. C. PA1 iv) Higher regenerator temperature requires better lining and metallurgy of the regenerator which increases the capital expenditure. PA1 a) introducing said catalyst and feed in a bottom riser and allowing the catalyst and feed to preaccelerate upwardly within said riser: PA1 b) the catalyst and hydrocarbon vapor mixture formed in said riser flowing upwardly through a plurality of microriser tubes disposed within a regenerator shell and so as to cause a cracking reaction of said hydrocarbon; PA1 c) allowing a simultaneous combustion of the coked catalyst within said regenerator and causing a regeneration of the catalyst and a heat transfer to said microriser tubes; PA1 d) the vapors from said microriser tubes passing through a catalyst separator and stripper; PA1 e) the spent coked catalyst being introduced into said regenerator.
These problems are further discussed in the following sections.